Under ordinary conditions it is considered that, in a vacuum, a laser pulse propagates without deformation and at the speed of light (c). In dispersive media, the pulse most often propagates at a speed slower than c, and its temporal profile varies over the course of propagation. However, by making use of media exhibiting abnormal dispersion, nonlinear behavior or gain, it is possible to decrease the speed of propagation of a pulse by a very large amount, or even to stop it, or conversely to make it much faster than c (without however breaking the rules of the theory of relativity: a detailed analysis shows that neither energy nor information propagates at a superluminous speed). See for example [1].
These superluminous or far-subluminous propagation speeds are obtained only within very particular optical media, such as atomic vapors, which limits the possible applications of these techniques and makes the implementation thereof complex.